Ambulance.
There are two telephone numbers in Beijing that can be dialed for medical emergencies: "120" for the Beijing Emergency Center and "999" for the Beijing Red Cross.
An ambulance should arrive within minutes and can be instructed to take patients to the nearest hospital or clinic, or to one of their choice. Another possibility is to call the emergency contact number of one of the hospitals serving foreigners (see Hospitals Link). This ensures that the person answering the phone will be able to speak English.
It's recommended for foreigners to keep insurance information and the address of a preferred hospital (in Chinese) on them at all times. And, of course, having a Chinese friend to assist with any interpreting most certainly wouldn't hurt.
Beijing Emergency Numbers:
120
999